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Old 4th Aug 2022, 5:53 pm   #6
murphyv310
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Join Date: Sep 2006
Location: Kilmarnock, Ayrshire, UK.
Posts: 5,088
Default Re: Band 1 Rod Pentode modulator (Part 2)

Hi.
Some interesting developments on the modulator this afternoon. For some reason I decided to reverse the grids on the vision modulator and lo and behold I don't need the positive bias to give me a positive going signal! The down side is it needs a lot more drive so its left as was.

Also I have modified the audio stage slightly by adding a 1K resistor between the earth side from the input audio transformer to ground bypassed by a 25μf cap and connecting the HT supply negative to the transformer, this giving the audio section of the 1ZH37B some negative bias (approx 1.95v) this increases the depth of modulation and reduces distortion. I think I'll be doing this mod to my 1ZH37B pantry transmitter.
I've updated the diagram.
The total HT current is a little below 2ma

PS There is a typo on post 1 The valves are NOT 1ZH42B, (which doesn't exist) they should be 1ZH37B, Sorry about the wrong info.
Attached Files
File Type: pdf rpentodech1upd.pdf (380.3 KB, 39 views)
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Trevor.
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